ON/OFF control in a burner is used specifically to

The use of ON/OFF control in a burner is specifically designed to start and stop the burner without any modulation of the flame. This method allows for a simple operational approach where the burner operates at full capacity when it is on and is completely off when the demand does not require heat. This binary operation is efficient for systems that can handle fluctuations in temperature or pressure without needing to fine-tune the flame intensity.

In contrast, other options suggest functionalities that go beyond the basic ON/OFF control. Modulating flame intensity according to steam demand involves gradually adjusting the burner output rather than simply turning it on or off. Continuous adjustment of air supply relates to maintaining optimal combustion conditions, which typically requires more sophisticated control systems. Automatic shutdown mechanisms in case of a failure imply a safety feature that is important for overall system safety but does not define the primary purpose of ON/OFF control itself. Therefore, the correct answer accurately reflects the fundamental characteristic of ON/OFF control in a burner application.